With change of possession and a single foul, you have three scenarios:
A fouls before COP -- B must decline to keep ball
B fouls before COP -- A's ball, enforce from previous spot (bean bag, end of run if fumble).
Either team fouls after COP -- pretend B was A the whole time, it's a running play, ABO applies.
If the COP was a scrimmage kick, then PSK may apply -- the goal of PSK is to transform a pre-COP B/R foul into a post-COP B/R foul where the "spot of the foul" (for enforcement purposes) is the end of the kick. Similarly, the 10-4-2 exception transforms a pre-COP A foul into a post-COP A foul during a scrimmage kick (although not mandatory, B/R can request a rekick). All of this is done to prevent "replay the down" when the down is a scrimmage kick.
If you have fouls by both teams, that's where things can get complicated, but things tend towards "double foul, reply down" if it can't be made simple. However, there's two basic scenarios here:
B fouls before COP -- double foul, replay the down.
B fouls after COP -- B must decline A's foul to keep ball, enforce B's penalty ABO (as above)
When you get more complicated things like multiple COPs, think clean hands (team in last possession cannot foul before getting the ball the last time) and team in last possession must decline other teams fouls -- if things get overly complicated, you probably don't have clean hands and it's "double foul, replay down".
UNS and Nonplayer are dead ball enforcement. This means you sort out all the live ball stuff unrelated to this, then start assessing the dead ball enforcement. There are some quirks (like equal numbers of UNSs cancel out and enforce the rest to prevent the 15 one way and half the distance the other), but nothing absurd.
